1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device.
2. Description of the Related Art
Among semiconductor devices which are currently mass-produced, there are semiconductor devices which, in order to improve functionality, are provided with a mixture of resistors and MOS transistors on the same substrate (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-119697). A resistor thereof is provided by, for example, implanting an impurity into a polysilicon layer formed on the substrate and then etching the polysilicon layer.
A conventional semiconductor device in which resistances and MOS transistors are formed on the same substrate will be described with reference to FIG. 10. FIG. 10 is a sectional view for describing the conventional semiconductor device.
An element isolation layer 124 is formed on a silicon substrate 122. A region at which this element isolation layer 124 is formed is referred to as an element isolation region 123. A MOS transistor region 121 is defined by the element isolation region 123. A gate oxide film 126 and a gate electrode 128 are layered, in that order, on the silicon substrate 122 within the MOS transistor region 121. A diffusion layer 129 is formed in the silicon substrate 122 within the MOS transistor region 121. The diffusion layer 129 has regional portions which are to function as a source and a drain of the MOS transistor, and is formed at locations sandwiching the gate oxide film 126 and gate electrode 128.
On the silicon substrate 122 at which the MOS transistor has been formed, a silicon oxide layer 130 is formed. The silicon oxide layer 130 is structured by laminating a mask oxide layer 132 and a resistance isolation oxide layer 134. A resistance 140 is then formed on the resistance isolation oxide layer 134 in the element isolation region 123. Thereafter, a silicon nitride layer 150 and an inter-layer insulation layer 160 are laminated, in that order, on the resistance isolation oxide layer 134 and the resistance 140.
The mask oxide layer 132 is employed as a mask when ion-implantation into the diffusion layer is being performed.
The resistance isolation oxide layer 134 is provided in order to prevent exposure of the silicon substrate 122 due to etching of the mask oxide layer 132 during etching for formation of the resistance 140. If the silicon substrate 122 were to be exposed, a region at which the silicon nitride layer 150 and the silicon substrate 122 touched would be formed, and an interface current would flow at the region at which the silicon nitride layer 150 and the silicon substrate 122 touched, which would be a cause of leaks.
A contact hole is formed penetrating through a laminate 165 of the silicon oxide layer 130, the silicon nitride layer 150 and the inter-layer insulation layer 160. A titanium conduction film 180 is formed on an inner wall face of the contact hole, and a contact plug 185 is formed over the titanium conduction film 180 so as to fill in the contact hole. The contact plug 185 is formed of, for example, tungsten. Here, at a lower portion of the contact hole, the titanium conduction film 180 takes the form of a titanium silicide 182.
In the conventional semiconductor device which has been described with reference to FIG. 10, a thickness t1 of the silicon oxide layer 130, which is structured by the mask oxide layer 132 and the resistance isolation oxide layer 134 being laminated, is around 90 nm at a region at which the contact hole is formed. Commonly, the mask oxide layer 132 is formed with a thickness of 20 nm and the resistance isolation oxide layer 134 is formed with a thickness of 70 nm. A thickness t2 of the laminate 165 of the mask oxide layer 132, the resistance isolation oxide layer 134, the silicon nitride layer 150 and the inter-layer insulation layer 160 is around 1050 nm.
In such a case, if the semiconductor device is left in the atmosphere for about 20 hours from after the contact hole is formed until when formation of the titanium conduction film 180, the titanium silicide 182 and the contact plug 185 is implemented, there is a problem in that anomalous leak currents occur in the semiconductor device and it cannot be used as a product.
A cause of anomalous leak currents which occur in the semiconductor device will be described with reference to FIG. 11. FIG. 11 is a diagram for describing formation of the titanium conduction film in the contact hole.
A contact hole 170 is ordinarily formed so as to penetrate through the laminate 165 by photolithography and dry etching. At such a time, because an etching rate of the silicon nitride layer 150 is lower than that of the silicon oxide layer 130, the silicon nitride layer 150 remains in a flange form at the inner wall face of the contact hole 170 (i.e., the portion indicated by I in FIG. 11). In this state, when formation of the titanium conduction film 180 is implemented by sputtering, the titanium conduction film is formed with sufficient thickness at a portion of the contact hole 170 to the upper side of the silicon nitride layer 150, that is, at a side wall of the inter-layer insulation layer 160. In contrast, at a portion to the lower side of the silicon nitride layer 150 (i.e., the portion indicated by II in FIG. 11) that is, at a side wall of the silicon oxide layer 130, the silicon nitride layer 150 impedes formation of the titanium conduction film 180, and the titanium conduction film cannot be formed with sufficient thickness.
If the semiconductor device is left in the atmosphere in this condition, the silicon substrate reacts with, for example, moisture in the air, and produces silicon oxides and the like. As a result, the formation of the titanium silicide at the lower portion of the contact hole is impeded, which leads to failures in conduction with a contact. In addition, p-n junction breakdown occurs in the diffusion layer, due to leaching of a wiring metal into the silicon substrate or the like, and as a result, leakage currents occur.
Therefore, ordinarily, a semiconductor device is fabricated by processing such that a period for which it is left in the atmosphere after contact holes have been formed is as short as possible.
In order to address the issue described above, the present inventors have carried out rigorous investigations, and have discovered that the portion of the titanium conduction film which is not formed with sufficient thickness at the lower portion of the contact hole can be reduced by making the film thickness of the silicon oxide layer 130 below the silicon nitride layer 150 thinner, and as a result, excellent titanium silicide can be formed.
It has also been discovered that an excellent titanium silicide can be formed by forming the inter-layer insulation layer and forming the contact hole after having implemented an opening in the silicon nitride layer, and thus preventing the flange-like remnant of the silicon nitride layer at the inner wall of the contact hole.